Why trees in the way
Fix for the given file


Made the adjustments @Delagone suggested: 


Separated the tree object
Set the object origins for each tree object root.
Made sure the three tree objects are in one group
Choose the group instead of the object in the "Render" tab.

In the "Emission" tab


Emit from: "Verts"
Unchecked "Random"





Distributing particle system with weight painting

Particle system source mesh

The mesh resolution of the plane is very low.

The weight painting is saved in the vertices of the mesh. The more control you want over the weight painting and therefore the distribution of the particle system, the more resolution the mesh needs.

Emission of the particle system



The emission from setting tells the particle system where it should emit the particles. In the case of a plane with weight paint it should be set to "Verts" or "Faces".

Weight painting

You want a road with trees on both sides.

Then you need to weight paint to lines of both sides of the street where the trees should be. 
The weight painting tells the particle system where it should put the trees. 

The weight paint tells the particle system the probability 

Example weight paints for road straight and with side street: